Transimpedance ampifier

ABSTRACT

Disclosed is a transimpendance amplifier comprising a single ended input terminal to receive an input signal from a photodiode and differential output terminals. A circuit coupled between the single ended input terminal and a differential output terminal may vary the gain of the transimpedance amplifier in response to a DC current component of the input signal.

The subject matter disclosed herein relates to U.S. patent applicationSer. No. 10/074,099, filed on Oct. 11, 2001, U.S. patent applicationSer. No. 10/074,397, filed on Feb. 11, 2002, and U.S. patent applicationSer. Nos. 10/324999, 10/324983, and Ser. No. 10/324048 filed on Dec. 20,2002.

BACKGROUND

1. Field

The subject matter disclosed herein relates to data communicationsystems. In particular, embodiments disclosed herein relate toprocessing data received from an optical transmission medium.

2. Information

Optical communication networks have been implemented to enable increaseddata rates in links providing point to point communication. For example,optical communication links are typically implemented in SynchronousOptical Network/Synchronous Digital Hierarchy (SONET/SDH) and 10 GigabitEthernet systems. At a receiving end of such an optical communicationlink, a photodiode may generate a current in response an optical signalreceived from an optical transmission medium (e.g., fiber opticalcabling). A transimpedance amplifier (TIA) typically converts thecurrent generated by the photodiode into a voltage signal that is thenprocessed. For example, the voltage signal may be processed by clock anddata recovery circuitry to recover data transmitted in the opticalsignal.

BRIEF DESCRIPTION OF THE FIGURES

Non-limiting and non-exhaustive embodiments of the present inventionwill be described with reference to the following figures, wherein likereference numerals refer to like parts throughout the various figuresunless otherwise specified.

FIG. 1 shows a schematic diagram of a system to transmit data in andreceive data from an optical transmission medium according to anembodiment of the present invention.

FIG. 2 shows a schematic diagram of physical medium attachment (PMA) andphysical medium dependent (PMD) sections of a data transmission systemaccording to an embodiment of the system shown in FIG. 2.

FIG. 3 shows a schematic diagram of a transimpedance amplifier (TIA)according to an embodiment of the PMD section shown in FIG. 2.

FIG. 4 shows a schematic diagram of a multistage amplifier according toan embodiment of the TIA shown in FIG. 3.

FIG. 5 shows a schematic diagram of a low pass filter circuit accordingto an embodiment of the TIA shown in FIG. 3.

DETAILED DESCRIPTION

Reference throughout this specification to one embodiment or anembodiment means that a particular feature, structure, or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, the appearances of the phrasein one embodiment or an embodiment in various places throughout thisspecification are not necessarily all referring to the same embodiment.Furthermore, the particular features, structures, or characteristics maybe combined in one or more embodiments.

A photodiode as referred to herein relates to a device that provides anoutput current in response to light energy collected on a surface. Forexample, a photodiode may provide an output voltage or an output currentin response to charge collected at a photodiode gate. However, this ismerely an example of a photodiode and embodiments of the presentinvention are not limited in these respects.

A transimpedance amplifier (TIA) as referred to herein relates to adevice to convert an input current to an output voltage. For example, aTIA may convert an input current received from a photodiode to an outputvoltage that is substantially proportional to a magnitude of the inputcurrent. However, this is merely an example of a TIA and embodiments ofthe present invention are not limited in this respect.

A single-ended terminal as referred to herein relates to an electricalterminal to transmit or receive a single-ended signal. For example,single-ended terminal may receive a signal as an input signal. However,this is merely an example of a single-ended terminal and embodiments ofthe present invention are not limited in this respect.

Differential terminals as referred to herein relates to a pair ofterminal that may receive or transmit a differential signal. Forexample, differential terminals signal may express a signal as a voltagedifference between the terminals. However, this is merely an example ofdifferential terminals and embodiments of the present invention are notlimited in this respect.

DC current as referred to herein relates to a current component in anelectrical signal that is substantially constant over a time period. Forexample, the current in a signal may comprise a DC current componentcombined with or added to an AC current component that fluctuates over atime period. However, this is merely an example of a DC current andembodiments of the present invention are not limited in these respects.

DC current detection circuit as referred to herein relates to a circuitthat is capable of detecting a DC current component in a signal. Forexample, a DC current detection circuit may generate a signal that isrepresentative of a magnitude of a DC current component in a signal.However, this is merely an example of a DC current detection circuit andembodiments of the present invention are not limited in this respect.

Briefly, an embodiment of the present invention relates to atransimpendance amplifier comprising differential output terminals and asingle ended input terminal to receive an input signal from aphotodiode. A circuit coupled between the single ended input terminaland a differential output terminal may vary a gain of the transimpedanceamplifier in response to a DC current component in the input signal.However, this is merely an example embodiment and other embodiments ofthe present invention are not limited in these respects.

FIG. 1 shows a schematic diagram of a system to transmit in and receivedata from an optical transmission medium according to an embodiment ofthe present invention. An optical transceiver 102 may transmit orreceive optical signals 110 or 112 in an optical transmission mediumsuch as fiber optic cabling. The optical transceiver 102 may modulatethe transmitted signal 110 or demodulate the received signal 112according to any optical data transmission format such as, for example,wave division multiplexing wavelength division multiplexing (WDM) ormulti-amplitude signaling (MAS). For example, a transmitter portion (notshown) of the optical transceiver 102 may employ WDM for transmittingmultiple lanes of data in the optical transmission medium.

A physical medium dependent (PMD) section 104 may provide circuitry,such as a TIA (not shown) and/or limiting amplifier (LIA) (not shown),to receive and condition an electrical signal from the opticaltransceiver 102 in response to the received optical signal 112. The PMDsection 104 may also provide to a laser device (not shown) in theoptical transceiver 102 power from a laser driver circuit (not shown)for transmitting an optical signal. A physical medium attachment (PMA)section 106 may include clock and data recovery circuitry (not shown)and de-multiplexing circuitry (not shown) to recover data from aconditioned signal received from the PMD section 104. The PMA section106 may also comprise multiplexing circuitry (not shown) fortransmitting data to the PMD section 104 in data lanes, and aserializer/deserializer (Serdes) for serializing a parallel data signalfrom a layer 2 section 108 and providing a parallel data signal to thelayer 2 section 108 based upon a serial data signal provided by theclock and data recovery circuitry.

According to an embodiment, the layer 2 section 108 may comprise a mediaaccess control (MAC) device coupled to the PMA section 106 at a mediaindependent interface (MII) as defined IEEE Std. 802.3ae-2002, clause46. In other embodiments, the layer 2 section 108 may comprise forwarderror correction logic and a framer to transmit and receive dataaccording to a version of the Synchronous Optical Network/SynchronousDigital Hierarchy (SONET/SDH) standard published by the InternationalTelecommunications Union (ITU). However, these are merely examples oflayer 2 devices that may provide a parallel data signal for transmissionon an optical transmission medium, and embodiments of the presentinvention are not limited in these respects.

The layer 2 section 108 may also be coupled to any of severalinput/output (I/O) systems (not shown) for communication with otherdevices on a processing platform. Such an I/O system may include, forexample, a multiplexed data bus coupled to a processing system or amulti-port switch fabric. The layer 2 section 108 may also be coupled toa multi-port switch fabric through a packet classification device.However, these are merely examples of an I/O system which may be coupledto a layer 2 device and embodiments of the present invention are notlimited in these respects.

The layer 2 device 108 may also be coupled to the PMA section 106 by abackplane interface (not shown) over a printed circuit board. Such abackplane interface may comprise devices providing a 10 Gigabit EthernetAttachment Unit Interface (XAUI) as provided in IEEE Std. 802.3ae-2002,clause 47. In other embodiments, such a backplane interface may compriseany one of several versions of the System Packet Interface (SPI) asdefined by the Optical Internetworking Forum (OIF). However, these aremerely examples of a backplane interface to couple a layer 2 device to aPMA section and embodiments of the present invention are not limited inthese respects.

FIG. 2 shows a schematic diagram of a system 200 to transmit data in andreceive data from an optical transmission medium according to anembodiment of the system shown in FIG. 2. An optical transceiver 202comprises a laser device 208 to transmit an optical signal 210 in anoptical transmission medium and a photo detector section 214 to receivean optical signal 212 from the optical transmission medium. The photodetector section 214 may comprise one or more photodiodes (not shown)for converting the received optical signal 212 to one or more electricalsignals to be provided to a TIA/LIA circuit 220. A laser driver circuit222 may modulate a modulation current 216 in response to a data signalfrom a PMA section 206. A laser device 208 may then modulate and powerthe transmitted optical signal 210 in response to the modulation current216.

According to an embodiment, the LIA portion of the PMD section mayprovide a conditioned signal to clock and data recovery (CDR) circuitry(not shown) in the PMA section 206. The LIA portion and the CDRcircuitry may be designed to process signals over a particular dynamicrange to enhance or reduce the bit error rate (BER) in the recovereddata. Such a dynamic range may be set as a system design parameter suchthat the signal provided to the CDR circuitry has sufficientsignal-to-noise ratio while accounting for saturation of detectioncircuitry for data signals approaching the upper and lower regions ofthe dynamic range.

On the other hand, a photodiode in the photodetector 214 may be designedto provide an output current that is specified according to systemparameters set forth in a particular one of various standards such asSONET/SDH and different Ethernet standards (e.g., for local areanetworks (LANs), wide area networks (WANs) and metropolitan areanetworks (MANs)). For example, the photodiode in the photodetector 214may be designed to provide an output current at a set dynamic range asprovided in a standard defined parameter that is specific to aparticular standard. According to an embodiment, the TIA/LIA section 220may be designed to provide an output to CDR circuitry in the PMA section202 at a dynamic range according to a system design parameter inresponse to a current from the photodetector 214 provided at any one ofseveral dynamic ranges (e.g., as provided in a particular standard).

FIG. 3 shows a schematic diagram of a transimpedance amplifier (TIA) 300according to an embodiment of the PMD section shown in FIG. 2. Anamplifier 302 receives a signal at a single-ended input terminal 304from a photodiode 306 which is responsive to an optical data signal andprovides a differential voltage signal at differential output terminals312 and 314. The TIA 300 may be formed as part of an integrated device(e.g., as part of a single device including the TIA 300 and otherportions of the PMD section) in a semiconductor process such as acomplementary metal oxide semiconductor (CMOS) manufacturing process.However, this is merely an example of a process that may be used to forma TIA and embodiments of the present invention are not limited in thisrespect.

FIG. 4 shows a schematic diagram of a multistage amplifier 402 accordingto an embodiment of the amplifier 302 shown in FIG. 3. A firstamplification stage comprises a transistor 406. A gate of the transistor406 may receive a single ended input signal on a single-ended inputterminal 404 from the photodiode 306. In response to the input signal, asecond amplification stage comprising a differential amplifier formed bytransistors 408 and 410 provides an output voltage on differentialoutput terminals 412 and 414. However, this is merely an example of anamplifier that may be used in a TIA to receive a single ended inputsignal and provide a differential output signal, and embodiments of thepresent invention are not limited in this respect.

According to an embodiment, the TIA 300 may provide an output voltage atthe output terminals in response to an input current received at thesingle-ended input 304 according to a signal gain. The output terminal312 may be coupled to the single ended input terminal 304 through aresistance section 308. According to an embodiment, increasing theresistance 308 in the feedback circuit may increase the signal gain ofthe TIA 300 while decreasing the resistance 308 may decrease the signalgain.

According to an embodiment, a DC current detection circuit may detect aDC component of the input signal provided by the photodiode 306. Aninput voltage V_(in) at the single-ended input terminal 304 issubstantially proportional to an output voltage V_(out) at an outputterminal 312. A voltage across the resistance 308 is substantiallyproportional to the magnitude of the current provided at thesingle-ended input 304. As such, the voltage across the resistance 308may have a DC component and an AC component that are substantiallyproportional to the magnitudes of respective DC and AC components of thecurrent at the input single on ended input terminal 304. Voltages at theterminals of the resistor 308 are provided to low pass filters (LPFs)316 and 318 to substantially remove the AC component of the voltageacross the resistor 308. Accordingly, the voltage between the outputs ofthe LPFs 316 and 318 may be substantially proportional to the DCcomponent in the voltage across the resistance 308. According to anembodiment, the LPFs 316 and 318 may be any LPF formed using a resistor510 and capacitor 512 as shown in FIG. 5. However, this is merely anexample of how a LPF may be formed in a circuit and embodiments of thepresent invention are not limited in these respects.

The outputs of the LPFs 316 and 318 may each be provided to acorresponding input terminal of an operational amplifier 320. Accordingto an embodiment, the magnitude of the output of the operationalamplifier 320 may be representative of the magnitude of the DC currentcomponent in the input signal received at the single-ended inputterminal 304. Buffer circuits 332 and 334 may control bypass transistors340 and 342 (coupled across respective resistors 322 and 324) to varythe magnitude of resistance 308 in response to the output of operationalamplifier 320. In the illustrated embodiment, the bypass transistors 340and 342 may decrease the resistance 308 as a function of the magnitudeof the DC current component in the input signal received at thesingle-ended input terminal 304. The bypass transistors 340 and 342 maysimilarly increase the resistance 308 as a function of the magnitude ofthe DC current component.

By varying the resistance 308 in response to the magnitude of the DCcurrent component, the gain of the TIA 300 may be varied in thatincreases in the resistance 308 may increase the gain of the TIA 300while decreases in the resistance 308 may decrease the gain of the TIA300. Accordingly, the gain of the TIA 300 may be set as a decreasingfunction of the magnitude of the DC current component in the inputsignal provided to the single-ended input 304.

According to an embodiment, the gain of the TIA 300 may be adjusted tomaintain a dynamic range of the output signal (at differential outputterminals 312 and 314) to meet system parameters defined for upstreamprocessing circuitry (e.g., CDR circuitry). Accordingly, the TIA 300 maybe adjusted for different requirements relating to LANs, WANs and MANs.Using techniques known to those of ordinary skill in the art of analogcircuit design, the resistances of resistors 322 and 324, gain ofoperational amplifier 320 and size of transistors 340 and 342 may beselected to maintain the dynamic range to within the system parametersbased upon the strength of the DC current component of the input signalon the single-ended input terminal 304.

According to an embodiment, an input capacitance at the single-endedinput terminal 304 may also be varied in response to changes in the gainof the TIA 300 (in response to detection of the DC current component inthe input signal provided to single-ended input terminal 304).Transistors 328 and 330, coupling respective capacitors 336 and 338 tothe single-ended input terminal 304, may control the input capacitancein response to the outputs of buffer circuits 332 and 334. Increases inthe input capacitance may maintain a phase margin that prevents orinhibits oscillation of the TIA 300. However, this is merely an exampleof how an input capacitance may be varied to maintain a phase margin,and embodiments of the present invention are not limited in theserespects.

According to an embodiment, a sink transistor 326 may removesubstantially all or a portion of the DC current component from theinput signal at the single-ended input terminal 304. As pointed outabove, the output voltage of the operational amplifier 320 may besubstantially proportional to the DC current component of the inputsignal provided to the single-ended input terminal 304. Accordingly, inresponse to the output voltage of the operational amplifier 320 appliedto a gate of the sink transistor 326, the sink transistor 326 may removeat least a portion of the DC current component from the input signal.Using techniques known to those of ordinary skill in the art of analogcircuit design, the gain of operational amplifier 320 and size oftransistor 326 may be selected such that current removed from thesingle-ended input 304 by the transistor 326 substantially removes theDC component of current at the single-ended input 304.

It should be understood that while the sink transistor 326 may removesubstantially all or a portion of the DC current component of the inputsignal, the outputs of the buffer circuits 332 and 334 may still controlthe gain and phase margin of the TIA 300 in response to detecting the DCcurrent component of the input signal. According to an embodiment, priorto removal of the DC current component by the sink transistor 326 thebuffer circuits 332 and 334 may receive a voltage from the output of theoperational amplifier 320 which is representative of the DC currentcomponent of the input signal. It should be appreciated that the voltageacross resistance 308 should be maintained about constant as the DCcurrent component received at the single-ended input terminal 304 isabout constant. The voltage across resistance 308 may change in responseto changes in the DC current component received at the single-endedinput terminal 304, causing a change in an amount of current drawn fromthe single-ended input terminal 304 by sink transistor 326.

By substantially removing the DC current component at the single-endedinput 304, downstream processing may more accurately recover datareceived from the photodiode 306. For example, removing the DC currentcomponent may better align an amplitude of an eye pattern signal to beprocessed by clock and data recovery circuitry in a PMA device,resulting in a reduced bit error rate.

While there has been illustrated and described what are presentlyconsidered to be example embodiments of the present invention, it willbe understood by those skilled in the art that various othermodifications may be made, and equivalents may be substituted, withoutdeparting from the true scope of the invention. Additionally, manymodifications may be made to adapt a particular situation to theteachings of the present invention without departing from the centralinventive concept described herein. Therefore, it is intended that thepresent invention not be limited to the particular embodimentsdisclosed, but that the invention include all embodiments falling withinthe scope of the appended claims.

What is claimed is:
 1. A transimpendance amplifier comprising: a singleended input terminal to receive an input signal from a photodiode; oneor more output terminals; a resistance comprising a plurality of seriescoupled resistors coupled between the single ended input terminal andone of said output terminals, each resistor having a componentresistance; a DC current detection circuit to detect a DC currentcomponent in the input signal; and a circuit to vary the componentresistance across one or more of said plurality of series coupledresistors in response to the detected DC current component.
 2. Thetransimpedance amplifier of claim 1, wherein the transimpedanceamplifier further comprises a circuit to vary an input capacitance atthe single ended input terminal in response to the detected DC currentcomponent.
 3. The transimpedance amplifier of claim 1, wherein the DCcurrent detection circuit comprises a circuit to detect a DC voltageacross the resistance.
 4. The transimpedance amplifier of claim 1,wherein the transimpedance amplifier further comprises a DC currentremoval circuit coupled to the single ended input terminal tosubstantially remove the DC current component.
 5. The transimpedanceamplifier of claim 4, wherein the DC current removal circuit comprises acurrent sink transistor coupled to the single ended input terminal toremove a current in response to the detected DC component.
 6. A systemcomprising: a photodiode; a transimpedance amplifier coupled to thephotodiode to provide a differential output signal; a data recoverycircuit to provide a serial data signal in response to the differentialoutput signal; and a deserializer to provide a parallel data signal inresponse to the serial data signal, wherein the transimpedance amplifiercomprises: a single ended input terminal to receive an input signal fromthe photodiode; one or more output terminals; a resistance comprising aplurality of series coupled resistors coupled between the single endedinput terminal and one of said output terminals, each resistor having acomponent resistance; a DC current detection circuit to detect a DCcurrent component in the input signal; and a circuit to selectively varythe component resistance across one or more of said plurality of seriescoupled resistors in response to the detected DC current component. 7.The system of claim 6, the system further comprising a SONET framer toreceive the parallel data signal.
 8. The system of claim 7, wherein thesystem further comprises a switch fabric coupled to the SONET framer. 9.The system of claim 6, the system further comprising an Ethernet MAC toreceive the parallel data signal at a media independent interface. 10.The system of claim 9, wherein the system further comprises amultiplexed data bus coupled to the Ethernet MAC.
 11. The system ofclaim 9, wherein the system further comprises a switch fabric coupled tothe Ethernet MAC.
 12. The system of claim 6, wherein the transimpedanceamplifier further comprises a circuit to vary an input capacitance atthe single ended input terminal in response to the detected DC currentcomponent.
 13. The system of claim 6, wherein the DC current detectioncircuit comprises a circuit to detect a DC voltage across theresistance.
 14. The system of claim 6, wherein the transimpedanceamplifier further comprises a DC current removal circuit coupled to thesingle ended input terminal to substantially remove the DC currentcomponent.
 15. The system of claim 14, wherein the DC current removalcircuit comprises a current sink transistor coupled to the single endedinput terminal to remove a current in response to the detected DCcomponent.
 16. A method comprising: receiving an input signal from aphotodiode at a single-ended input terminal of a transimpendanceamplifier, transmitting an output signal from one or more terminals ofthe transimpedance amplifier; detecting a DC current component in theinput signal; and varying one or more component resistances of aplurality of series coupled resistors coupled between the input terminalin response to the detected DC current component.
 17. The method ofclaim 16, the method further comprising varying an input capacitance atthe single ended input terminal in response to the detected DC currentcomponent.
 18. The method of claim 16, wherein detecting the DC currentcomponent in the input signal further comprises: measuring a DC voltageacross the plurality of series coupled resistors; and generating avoltage representative of the DC current component in response to the DCvoltage across the of series coupled resistors.
 19. The method of claim16, the method further comprising removing at least a portion of the DCcurrent component from the single-ended input terminal.